What causes the rear of the truck to slope?

[Candy]

I have a 99 TR4 SR5 that I just got this weekend. I love the truck. It's my first SUV ever and she's gorgeous (I'll post pics later), but my question is I've always noticed other TR4's that slope in the back... what causes this and how can you fix it? My truck looks like it's sloping but maybe it's just me, I don't know!

 

[Grimsta]

Spring sag. Its a biiiiiiig 4Runner problem. I've replaced many springs on 4R's that do this.